Twenty adolescents, fitted with intraoral fluoride releasing devices (IFRD), were monitored over a six month period to determine if there were certain microbiological changes in the plaque at the attachment site of the IFRD. Increases in the percentage of S. mutans and total cultivable flora were observed in supragingival plaque after placement of the IFRD. There was also an increase in the proportions of motile rods and spirochetes in the subgingival plaque.